IEEE 802.16 is a series of wireless broadband standards developed by the IEEE (Institute of Electrical and Electronic Engineers) and is incorporated herein fully by reference. IEEE 802.16e uses OFDMA (Orthogonal Frequency Division Multiple Access) to carry data. It supports adaptive modulation and coding so that, when channel quality conditions are relatively good, more efficient, but less robust coding schemes may be used, e.g., 64 QAM (Quadrature Amplitude Modulation), so as to maximize the total number of channels or data that can be transmitted per unit time. However, when channel quality conditions are relatively poor, less efficient, but more robust modulation and coding schemes may be used, e.g., BPSK (Binary Phase Shift Keying), so as to increase channel quality (but at the cost of lower data transmission rates). Any number of different modulation schemes may be supported depending on channel conditions. For instance, under the best conditions, a 64 QAM modulation scheme may be selected, and, under the worst conditions a BPSK modulation scheme may be selected. When channel conditions are between the two extremes, other intermediate modulation schemes having intermediate compromises between efficiency and robustness may be selected, such as 16 QAM, QPSK, etc.
IEEE 802.16 also supports Hybrid Automatic Repeat Request (HARQ) for improved error detection and correction performance.
Another feature of IEEE 802.16 is MIMO operation. MIMO is a feature in which the transceivers in the network each have multiple transmit and/or receive antennas so as to permit directional beam patterns that can be aimed by the transmitter in the direction of the intended receiver and/or vice versa in order to improve signal strength to the intended receiver without increasing power at the transmitter. It also tends to decrease interference between channels and improve NLOS (Non-Line-Of-Sight) characteristics.
Single-user MIMO, or SU-MIMO, refers to use of MIMO in which only one transmitter and receiver communicate at any given time on any given communication resource unit. Multiple-User MIMO, or MU-MIMO, is a technique by which a transmitter, e.g., a base station, uses MIMO to simultaneously transmit two different signals to two different receivers in the same communication resource unit. More particularly, since the transmitter can beamform the transmission beams, it can direct the data/signals intended for one receiver (e.g., mobile station) toward that receiver and direct the data/signals intended for another receiver toward the other receiver. If the two beams are directed in sufficiently different directions and the two receivers are sufficiently far apart from the transmitter such that, at each receiver, the data signals intended for that receiver are substantially stronger than the data/signals intended for the other receiver operating on the communication resource unit, the two receivers can share the same channel.
It is well known that, in OFDM, each downlink communication channel between a base station and a mobile station actually comprises a certain subset of sub-carrier frequencies and a certain subset of time slots (e.g., within a frame) in the overall time and frequency spectrum available to the network. As described above, in MU-MIMO, the base station transmits to two (or more) mobile stations within the same subsets of timeslots and frequencies. In other words, in MU-MIMO, the base station is supporting two distinct communication channels on the same subset of timeslots and frequencies. That is why, in order to avoid confusion in this specification, the term “communication resource unit” is used herein to refer to a given subset of timeslots and frequencies; and the term “communication channel” is used to refer to an individual downlink with an individual mobile station.
Networks that support MU-MIMO operation often will operate in SU-MIMO until the call load exceeds a certain threshold; and then switch some or all of the calls to MU-MIMO operation in order to support a greater number of simultaneous calls, albeit of lower quality. In some networks, when switching from SU-MIMO operation to MU-MIMO operation, the communication resource units may remain the same size (i.e., occupy the same number of sub-carrier-frequencies and timeslots per unit time) and are just shared by two (or more) mobile stations. However, in some networks, the communication resource units for MU-MIMO operation may be a different size (presumably larger—occupying more sub-carrier frequencies and/or timeslots per unit time) than the SU-MIMO communication resource units so as to lessen the degradation in the communication quality for each mobile station sharing the MU-MIMO communication resource unit, but at the expense of a smaller increase in overall call capacity.
In wireless communication networks having adaptive modulation and coding (AMC), such as IEEE 802.16 networks, it is typically the base stations that decide which modulation and coding schemes to use. As mentioned above, such decisions are commonly based on some measurement of the quality of the channel, and particularly the downlink channel. Since a base station cannot directly measure the quality of its own downlink channels, the mobile stations are adapted to determine the downlink channel quality, such as by observing the pilot bits in the downlink channel, the bit error correction rates on the received data, etc., and then transmit channel quality data back to the base station on the control channel. For instance, in IEEE 802.16, the mobile stations transmit a parameter called CQI (Channel Quality Index) to the base station. There are several techniques for generating and transmitting CQI in IEEE 802.16, including codebook-based CQI and sounding-based CQI. In codebook-based CQI, there is a limited number of possible CQI values, each CQI value corresponding to a channel condition. Both the mobile station and base station have the codebook that discloses the meaning of each of those possible CQI values. The mobile station sends CQI value to the base station requiring very little bandwidth and the base station plugs that CQI value into the codebook to determine what it signifies. For instance, the codebook may reveal, among other things, that a CQI value of 4 means that the base station should select a modulation scheme of 16 QAM with a ½ coding rate.
Nevertheless, when in MU-MIMO mode, it may be necessary to transmit a great deal of CQI data per mobile station because the mobile stations do not have any information about the other mobile stations with which they may be sharing a communication resource unit.